262  PRE-C  AM  BRIAN   GEOLOGY    OF    NORTH   AMERICA. 
the  edges  of  the  crystalline  Huronian  schists  and  are  cut  by  great 
dikes  of  diorite.  The  great  Keweenaw  group,  with  its  cupriferous. 
amygdaloids,  is  here  absent,  although  met  a  few  miles  to  the  east. 
This  group,  as  shown  by  Brooks  and  Pumpelly,  occupies  a  place  be- 
tween  the  Huronian  schists  and  the  nearly  horizontal  red  and  white 
sandstone  of  the  region,  which  is  itself  below  the  Trenton  limestone. 
Bell,23g  in  1874,  in  the  country  between  the  Red  River  and  the 
South  Saskatchewan,  finds  extensive  mica  schists  and  gneisses;  but  a 
broad  band  of  schist,  having  the  character  of  the  Huronian  forma- 
tion, crosses  the  central  part  of  Rainy  Lake.  On  the  islands  of  the 
Lake  of  the  Woods  the  granites,  gneisses,  and  Huronian  schists  are 
intricately  mingled  with  one  another. 
Dawson  (G.  M.),237  in  1875,  gives  an  account  of  the  geology  of  the 
Lake  of  the  Woods,  where  the  rocks  are  wholly  Laurentian  and 
Huronian.  The  Laurentian  formation  is  represented  by  a  great 
thickness  of  granitoid  and  thick-bedded  gneisses,  which  pass  upward 
into  thin-bedded  gneisses  and  highly  crystalline  micaceous  and  horn- 
blenclic  schists. 
The  Huronian  rocks  are  more  variable  in  character.  The  lowest 
beds  are,  for  the  most  part,  hard  green  rocks,  with  little  traces  of 
stratification,  but  hold  some  well-stratified  micaceous  and  chloritic 
schists  and  also  imperfect  gneiss.  On  these  rests  a  great  thickness  of 
massive  beds  characterized  by  the  predominance  of  conglomerate,  but 
including  quartzites  and  dioritic  rocks.  Above  these  is  an  extensive 
series  of  schistose  and  slaty  beds,  generally  more  or  less  nacreous  and 
chloritic  or  talcose,  but  often  hornblendic  and  micaceous.  They  in- 
close also  conglomerates,  quartzites,  and  diorite  beds.  It  is  believed 
that  two  movements  have  conspired  to  form  the  present  features  of 
the  region,  both  being  post-Huronian.  The  first  of  these  is  connected 
with  the  post-Huronian  granite  eruptions;  the  second  and  more  im- 
portant is  believed  to  have  taken  place  later,  and  to  it  is  supposed  to 
be  due  the  parallelism  in  the  folding  of  the  Laurentian  and  Huronian 
rocks.  At  Rat  Portage  the  junction  of  the  Laurentian  and  Huronian 
is  so  sharply  defined  that  the  hand  can  be  laid  upon  it.  This  sharp 
contact  is  believed  to  be  due  to  faulting.  Adjacent  to  the  granite  the 
Huronian  slate  series  is  metamorphosed,  and  the  occasional  gneissic 
aspect  of  the  Huronian  is  attributed  to  the  granitic  intrusions.  The 
large  Y-shaped  granite  mass  in  the  northwest  angle,  in  contact  with 
the  altered  sedimentary  rocks,  assumes  a  more  basic  character  and  a 
darker  aspect,  becoming  blackish  gneissic  diorite  and  gray  syenitic 
diorite.  The  conglomerate  beds  are  of  immense  thickness  and  could 
perhaps  be  best  described  as  slate  conglomerates.  The  pebbles  gener- 
ally resemble  the  matrix,  and  best  appear  upon  a  weathered  surface; 
for  on  a  freshly  broken  surface  no  clear  distinction  appears  between 
the  fragments  and  the  inclosing  materials,  and  the  rock  differs  from 
